During the reversal of renal hypertension (removal of a constricting clip from the renal artery) arterial pressure returns to normotensive level within 6 to 24 h. This rapid reversal is accompanied by a brisk diuresis and it has been suggested that a decrease in blood volume resulting from the diuresis is responsible for the decrease in arterial pressure. It has also been suggested that plasma fluid leaves the circulation by a transfer of fluid into the tissues due to a primary decrease in interstitial fluid pressure during the reversal of renal hypertension. Experiments will be conducted to measure the rapid (2-6 h.) change in blood volume and interstitial fluid pressure during the reversal of renal hypertension in rats. Since the ratio of sodium to potassium intake may be important in the regulation of arterial pressure other experiments will be conducted to determine the effect of altered potassium in the diet in rats subjected to subtotal nephrectomy plus increased sodium intake. Most previous studies have been aimed at the volume effects of increased sodium intake in animals with subtotal nephrectomy hypertension. However, in other models of high-sodium hypertension which were previously believed to be "volume" models the role of the sympathetic nervous system has been found to be increasingly important. These experiments will also be conducted to determine the effects of sympathectomy on the development of subtotal nephrectomy plus high sodium hypertension in rats.